Low-carbohydrate
diets
or low-carb diets are dietary programs that restrict carbohydrate
consumption usually for weight control. Foods high in digestible carbohydrates
are limited or replaced with foods containing a higher percentage of
proteins
and fats.

The precise definition
of low-carbohydrate diets varies greatly. The term is most
commonly used to refer to ketogenic diets, i.e. diets that
restrict carbohydrate intake sufficiently to cause ketosis like the
Atkins diet,[1][2][3] but some sources consider less restrictive
variants to be low-carbohydrate as well.[4]

Apart from obesity
low-carbohydrate diets are often discussed as treatments for some
other conditions, most notably diabetes[5][6][7] and epilepsy,[8][9][10] although these treatments still remain
controversial and lack widespread support.

History

Beginnings

In 1863 William
Banting, an obese English undertaker and coffin maker, published "Letter
on Corpulence Addressed to the Public" in which he described a diet
for weight control giving up bread, butter, milk, sugar, beer and
potatoes.[11] His booklet was widely read, so much so that
some people used the term "Banting" for the activity usually called
"dieting."[12].

In 1967, Dr. Irwin
Stillman published The Doctor's Quick Weight Loss Diet. The "Stillman
Diet" is a high-protein, low-carbohydrate and low-fat diet. It is
regarded as one of the first low-carbohydrate diets to become popular
in the US.[13] Other low-carbohydrate diets in the
1960's included Air Force Diet[14] and the Drinking Man’s Diet.[15]
Austrian physician Dr Wolfgang Lutz published his book 'Leben Ohne
Brot' (Life Without Bread) in 1967.[16]
However it was hardly noticed in the English speaking world.

In 1972, Dr. Robert
Atkins published Dr. Atkins Diet Revolution which advocated a low-carbohydrate
diet he had successfully used in treating patients in the 1960s (having
himself developed the diet from an unspecified article published in
JAMA).[17]
The book met with some success but, because of research at that time
suggesting risk factors associated with excess fat and protein, it
was widely criticized by the mainstream medical community as being
dangerous and misleading, thereby limiting its appeal at the time.[18]
Among other things critics pointed out that Dr. Atkins had done little
real research into his theories and based them mostly on anecdotal
evidence.

The concept of
the glycemic index was invented in 1981 by Dr. David
Jenkins. This concept evaluates foods according to their insulin demand
-- with fast digesting simple carbohydrates having a high insulin
demand and slower digesting complex carbohydrates such as grains having
a lower insulin demand.[19]

Low
carb craze

In
the 1990s Dr. Atkins published Dr. Atkins New Diet Revolution and other doctors began
to publish books based on the same principles. This has been said
to be the beginning of the "low carb craze."[20] During
the late 1990s and early 2000s low-carbohydrate diets became some
of the most popular diets in the U.S. (by some accounts as much as
18% of the population was using a low-carbohydrate diet at its peak[21]) and spread to many countries. These were, in fact,
noted by some food manufacturers and restaurant chains as substantially
affecting their businesses (notably Krispy Kreme[22]).
This was in spite of the fact that the mainstream medical community
continued to denounce low-carbohydrate diets as being a dangerous
trend.[23][24][25] It is, however, valuable to note that
many of these same doctors and institutions at the same time quietly
began altering their own advice to be closer to the low-carbohydrate
recommendations (e.g. eating more protein, eating more fiber/less
starch, reducing consumption of juices by children)[26]. The
low-carbohydrate advocates did some adjustments of their own increasingly
advocating controlling fat and eliminating trans fat.[27][28] Many of the diet guides and gurus
that appeared at this time intentionally distanced themselves from
Atkins and the term low carb (because of the controversies)
even though their recommendations were based on largely the same principles
(e.g. the Zone diet).[29][30] As such it is often a matter of debate
which diets are really low-carbohydrate and which are not. The 1990s
and 2000s also saw the publication of an increased number of clinical
studies regarding the effectiveness and safety (pro and con) of low-carbohydrate
diets (notably a 2006 NEJM paper by Halton et al. describing a 20-year
study).

After 2004 the
popularity of this diet trend began to wane significantly although
it still remains quite popular. In spite of the decline in popuarlity
this diet trend has continued to quietly garner attention in the medical
and nutritional science communities.[31][32][33][34]

Practices
and theories

The term low-carbohydrate
diet today is most strongly associated with the Atkins Diet. However, there is an array of other diets
that share to varying degrees the same principles (e.g. the Zone Diet, the Protein Power Lifeplan, and the South Beach Diet).[35] Therefore, there is no widely accepted
definition of what precisely consistutes a low-carbohydrate diet.
It is important to note that the level of carbohydrate consumption
defined as low-carbohydrate by medical researchers may be different
than the level of carbohydrate defined by diet advisors. For the purposes
of this discussion, we focus on diets that reduce (nutritive) carbohydrate
intake sufficiently to dramatically reduce or eliminate insulin production
in the body and to encourage ketosis (production of ketones to be
used as energy in place of glucose).

Although originally
low-carbohydrate diets were created based on anecdotal evidence of
their effectiveness, today there is a much greater theoretical basis
on which these diets rest.[36][37] The key scientific principle which forms
the basis for these diets is the relationship between consumption
of carbohydrates and their effects on blood sugar (i.e. blood glucose)
and hormone production. Blood sugar levels in the human body must
be maintained in a fairly narrow range to maintain health. The two
primary hormones related to regulating blood sugar levels, produced
in the pancreas, are insulin, which lowers blood sugar levels, and
glucagon, which raises blood sugar levels.[38]
In general, most western diets (and many others) are sufficiently
high in nutritive carbohydrates that virtually every meal causes substantial
insulin production and avoids ketosis, thus causing excess energy
in the diet to be stored as fat (discussed in the next section). By
contrast, low-carbohydrate diets, or more properly, diets that are
very low in nutritive carbohydrates, discourage insulin production
and tend to cause ketosis. Some researchers suggest that this causes
excess dietary energy and body fat to be eliminated from the body.
Although these diets remain controversial, clinical studies show that
"Participants on a low-carbohydrate diet had more favorable overall
outcomes at 1 year than did those on a conventional diet."[39][40]

Low-carbohydrate
diet advocates in general recommend reducing nutritive carbohydrates
(commonly referred to as "net carbs," i.e. grams of total carbohydrates
reduced by the non-nutritive carbohydrates)[41][42]
to very low levels. This means sharply reducing consumption of desserts,
breads, pastas, potatoes, rice, and other sweet or starchy foods.
Some recommend levels as low as 20-30 grams of "net carbs" per day,
at least in the early stages of dieting[43]
(for comparison, a single slice of white bread typically contains
15 grams of carbohydrate, almost entirely starch). By contrast, more
standard nutrition guides typically recommend consumption levels in
the neighborhood of 225-325 grams of carbohydrate per day (based on
a 2000 calorie a day diet).[44][45]
Low-carbohydrate diets often differ in the specific amount of carbohydrates
allowed, whether certain types of foods are preferred, whether occasional
exceptions are allowed, etc. Generally they all agree that processed
sugar should be eliminated, or at the very least greatly reduced,
and similarly generally discourage heavily processed grains (white
bread, etc.). They vary greatly in their recommendations as to the
amount of fat allowed in the diet although the most popular versions
today (including Atkins)
generally recommend at most a moderate fat intake.

As a related note,
there is a set of diets known as low-glycemic-index diets (low-GI diets) or low-glycemic-load
diets (low-GL diets), in particular the Low GI Diet by Brand-Miller
et al.[46] In reality, low-carbohydrate
diets are, literally speaking, low-GL diets (and vice versa) in that
they specifically limit what contributes to the glycemic load in foods.
In practice, though, "low-GI"/"low-GL" diets differ from "low-carb"
diets in the following ways. First, low-carbohydrate diets treat all
nutritive carbohydrates as having the same effect on metabolism and
generally assume that their effect is independent of other nutrients
in food. Low-GI/low-GL diets base their recommendations on the actual
measured metabolic (glycemic) effects of the foods eaten. Second,
as a practical matter, low-GI/low-GL diets generally do not recommend
diets with glycemic loads low enough to minimize insulin production
and induce ketosis, whereas low-carbohydrate diets generally do.

Another related
diet type, the low-insulin-index diet, is very similar except that
it is based on measurements of direct insulemic responses (i.e. the
amount of insulin in the bloodstream) to food rather than glycemic
response (the amount of glucose in the bloodstream). Although the
diet recommendations mostly involve lowering nutritive carbohydrates,
there are some low-carbohydrate foods that are discouraged as well
(e.g. beef).[47]

In contrast to
these diets, based on evidence for risk of heart disease and obesity,
the Institute of Medicine recommends that American and Canadian adults
get between 40-65% of dietary energy from carbohydrates.[48]
The Food and Agriculture Organization and World Health Organization
jointly recommend that national dietary guidelines set a goal of 55-75%
of total energy from carbohydrates, but only 10% should be from Free
sugars (their definition of simple carbohydrates).[49]

Ketosis
and insulin synthesis: what is normal?

At the heart of
the debate about most low carbohydrate diets are fundamental questions
about what is a "normal" diet and how the human body is supposed to
operate. These questions can be summarized as follows. Nutritive carbohydrates
(starches and sugars) in the diet tend to break down very easily into
glucose in the bloodstream (blood sugar) when consumed. Glucose in
the blood is used by the cells in the body for energy for their basic
function. Excessive amounts of glucose in the blood are toxic to the
human body (the reason diabetes causes such serious health problems).
In general, unless a meal is very low in starches and sugars the level
of glucose will tend to rise to potentially dangerous levels. When
this occurs, the pancreas automatically produces insulin to cause
the liver to convert glucose into glycogen (glycogenesis) and triglycerides
(which can become body fat), thus reducing the blood sugars to safe
levels.[50] Diets with a
high starch/sugar content, therefore, cause sharp spikes in insulin
production. As such the blood sugar levels are highly variable with
every meal.

By contrast, if
the diet is very low in starches and sugars (low-carbohydrate diets)
the blood sugar level can fall so low that there is insufficient glucose
to fuel the cells in the body. This state causes the pancreas to produce
glucagon.[51] Glucagon causes the conversion of stored
glycogen to glucose and, once the glycogen stores are exhausted, causes
the liver to synthesize ketones (ketosis) and glucose (gluconeogenesis)
from fats and proteins, respectively. Most cells in the body can use
ketones for energy instead of glucose, and since ketones are easier
to produce, only a small amount of glucose is created (in other words,
ketosis is the more significant process in this case). Because diets
low in starches and sugars do not tend to directly affect blood sugar
levels significantly, meals tend to have little direct effect on insulin
levels (and so such diets tend to discourage insulin production in
general).

The diets of most
people in modern, so-called western nations, especially the United
States contain significant amounts of starches (and, frequently, significant
amounts of sugars). As such, the metabolisms of most westerners tend
to operate outside of ketosis and tend to involve significant insulin
production. This has been regarded by medical science in the last
century as being "normal." Ketosis has generally been regarded as
a dangerous (potentially life-threatening) state which unnecessarily
stresses the liver and causes destruction of muscle tissues. The view
that has been developed is that getting energy more from protein than
carbohydrates causes liver damage and that getting energy more from
fats than carbohydrates causes heart disease and other health problems.
This view is still the view of the majority in the medical and nutritional
science communities.[52][53][54]

Most advocates
of low-carbohydrate diets (specifically those that recommend diets
similar to the Atkins Diet) argue that this metabolic state (using
primarily blood glucose for energy) is not normal at all and that
the human body is, in fact, supposed to function primarily in ketosis.[55][56] They argue that high insulin levels
can, in fact, cause many health problems, most significantly, fat
storage and weight gain. They argue that the purported dangers of
ketosis are unsubstantiated (some of the arguments against ketosis
result from confusion between ketosis and ketoacidosis which is a
related but very different process).[57]
They also argue that fat in the diet only contributes to heart disease
in the presence of high insulin levels and that if the diet is instead
adjusted to induce ketosis, fat and cholesterol in the diet are not
a major concern (although most do not advocate unrestricted fat intake
and do advocate avoiding trans
fat). Further, whereas insulin in the bloodstream causes storage
of food energy, when the body is in ketosis, excess ketones (which
contain excess energy) are excreted in the urine and the breath.[58]

This debate is
on-going and no consensus currently exists.

Scientific
research

Because of the
substantial controversy regarding low-carbohydrate diets and even
disagreements in interpreting the results of specific studies it is
difficult to objectively summarize the research in a way that reflects
scientific consensus.[59][60] Although there has been some research
done throughout the twentieth century, most directly relevant scientific
studies have occurred in the 1990s and early 2000s and, as such, are
relatively new. Contrary to popular belief that low-carbohydrate diets
damage the heart, one study found that women eating low-carbohydrate,
high-fat/protein diets had the same or slightly less risk of coronary
heart disease, compared to women eating high-carbohydrate, low-fat
diets.[61] Other studies have found possible benefits
to individuals with diabetes,[62] cancer,[63][64] and autism.[65]
The ketogenic diet, with 90% of energy from fat and much of the remaining
from protein, has been used since the 1920s to treat epilepsy.[66] The introduction of modern anticonvulsant drugs,
however, substantially restricted its use. Interestingly, there has
very recently been renewed interest in use of the diet, especially
in children.[67]

A study conducted
in 1965 at the Oakland (California) Naval Hospital used a diet of
1000 calories per day, high in fat and limiting carbohydrates to 10
grams (40 calories) daily. Over a ten-day period, subjects on this
diet lost more body fat than did a group who fasted completely. (Benoit
et. al. 1965). Some subsequent studies have shown similar results.
Many advocates of low-carbohydrate diets have termed this the metabolic
advantage of such diets although many experts dispute whether
this is truly a general phenomenon.[68][69][70] Among others, recent studies from Stanford
University (2007) and Duke University (2005) comparing various diets
seem to favor low-carbohydrate diets for both weight loss and health
indicators.[71][72]

Criticism
and controversies

Water-related
weight loss

In the first week
or two of a low-carbohydrate diet a great deal of the weight loss
comes from eliminating water retained in the body (many doctors say
that the presence of high levels of insulin in the blood causes unnecessary
water retention in the body[73]).
However, this is a short-term effect and is entirely separate from
the general weight loss that these diets can produce through eliminating
excess body fat.

Exercise

Some critics argue
that low-carbohydrate diets can inherently cause weakness or fatigue[74]
giving rise to the occasional assumption that low-carbohydrate dieting
cannot involve an exercise regimen. Advocates of low-carbohydrate
diets generally dispute any suggestion that such diets cause weakness
or exhaustion (except in the first several days as the body adjusts)
and indeed most highly recommend exercise as part of a healthy lifestyle.[73]

Micronutrients
and vitamins

The major low-carbohydrate
diet guides generally recommend multi-vitamin and mineral supplements
as part of the diet regimen which may lead some to believe that these
diets are nutritionally deficient. The primary reason for this recommendation
is that if the switch from a high-carbohydrate to a low-carbohydrate,
ketogenic diet is rapid, the body can temporarily go through a period
of adjustment during which the body may require extra vitamins and
minerals (the reasons have to do with the body's releasing excess
fluids that were stored during high-carbohydrate eating). In other
words, the body goes through a temporary "shock" if the diet is changed
to low-carbohydrate dieting quickly just as it would changing to a
high-carbohydrate diet quickly. This does not, in and of itself, indicate
that either type of diet is nutritionally deficient. Some critics
have argued or implied that "carbohydrates contain vitamins" and minerals[75] but this suggestion is strictly false (by definition).
While it is true that many foods that are rich in carbohydrates are
also rich in vitamins and minerals, there are many low-carbohydrate
foods that are similarly rich in vitamins and minerals.[76]
Also, the important vitamin B12 is only available in significant quantities
from animal sources and not from vegetable sources.[77]

It should be noted
that, contrary to the recommendations of most diet guides, some individuals
may choose to avoid vegetables altogether in order to minimize carbohydrates.
It is more likely that such a diet could be nutritionally deficient
(some would dispute this based on cases like Vilhjalmur Stefansson).

Other
controversies

In 2004, the Canadian
government ruled that foods sold in Canada could not be marketed with
reduced or eliminated carbohydrate content as a selling point because
reduced carbohydrate content was not determined to be a health benefit,
and that existing "low carb" and "no carb" packaging would have to
be phased out by 2006.

Some variants
of low carbohydrate diets involve substantially lowered intake of
dietary fiber which can result in constipation if not supplemented.
For example, this has been a criticism of the Induction stage of the
Atkins diet (note that today the Atkins diet is more clear about recommending
a fiber supplement during Induction). Most advocates today argue that
fiber is a "good" carbohydrate and in fact encourage a high-fiber
diet.

It has been hypothesized
that a diet related change in blood acidity can lead to bone loss
through a process called ketoacidosis, as mentioned earlier in this
article. However ketoacidosis, which is often confused with ketosis,
is an acute medical condition caused by extreme fasting or as a symptom
of untreated diabetes, and is not likely to be induced by a proper
low-carbohydrate diet.

One of the occasional
side effects of a ketogenic diet is a noticeable smell of ketones
in the urine, perspiration, and breath.[78] This is caused by the temporary metabolism of fatty-acid
derived acetyl-coa into the ketone form, so that it may be released
from the liver into the blood stream. The ketones are then re-assembled
when they reach various body tissue's to form acetyl-coa again, which
is used as the precursor to energy.

Interview
with Charles H. Perle, DMD - "Basically when you are on a low-carbohydrate
diet, the key to success is breaking fat into ketones to create
ketosis, and as ketones get into urine and saliva, it can cause
horrible breath,"; WebMD feature; "Low-Carb Diets Can Cause Bad
Breath"; http://www.medicinenet.com/script/main/art.asp?articlekey=52504